The phase inversion method of making cellulose ester membrane is known, and generally comprises preparing a homogeneous casting solution of the cellulose ester in a solvent or solvent mixture which may include various additives, then casting the solution into a film, followed by immersion in a so-called "non-solvent" precipitant solution at ambient to subambient temperatures to precipitate the membrane. The basic requirements of the non-solvent precipitant solution are that it be a poor solvent for the cellulose ester and it be miscible with the casting solvents. Additives may also be included in the non-solvent solution to modify its solvating power for the cellulose ester and to control its miscibility with the casting solvents. The cellulose ester membrane may then be converted to a highly solvent-resistant regenerated cellulose membrane by hydrolysis with an aqueous solution of a strong base such as sodium hydroxide which contains a highly polar solvent such as dimethylsulfoxide (DMSO).
Cellulose triacetate (CTA) is a desirable membrane material because it forms strong films that are inert toward many solvents. There are few reported methods for the manufacture of CTA membranes. This is undoubtably due to the limited choice of solvents for CTA. Reverse osmosis (RO)-type CTA membranes have been prepared using mixtures of acetone with dioxane or DMSO as the casting solvents. Other components are later added to the solution just prior to casting, and the cast films are then precipitated in a subambient water bath containing still more additives. There is therefore a need in the art for a simple method of preparing CTA membranes without the need for a complex series of additives.
Recently a need has arisen for new porous membranes that can serve as supports for composite immobilized liquid membranes useful in facilitated transport processes such as for the production of oxygen using oxygen carriers in such liquid membranes. See, for example, the review by Way, et al., entitled "Selection of Supports for Immobilized Liquid Membranes" In Materials Science of Synthetic Membranes, ACS Symposium Series No. 269, pages 119-128 (1985). The support membrane greatly influences the performance of the resulting facilitated-transport liquid membrane. The support membrane must be chemically inert toward the components in the liquid membrane. The support must also be highly porous and have a pore diameter large enough to allow free diffusion of the carrier, yet not have pores so large that the liquid membrane is not effectively retained within them. Finally, the support membrane should be thin because the flux across a membrane is inversely proportional to its thickness.
There has also been a long-felt need in the dialysis art for suitably strong and highly permeable membranes capable of removing salts and low molecular weight organic compounds.
Further, there has been a need for such membranes for the manufacture of ion-selective electrodes used in analytical applications and as the electrode barrier for selective metal separation electrolytic processes.
These needs and others are met by the process and product of the present invention, which are summarized and described in detail below.